Paper handling specifies the method(s) used to transport paper through the printer. There are two classical techniques for transporting paper through a printer; tractor-feed and friction-feed. The tractor-feed is the most widely accepted type of mechanism for use in general-purpose or high-volume printing, especially for impact dot-matrix printers, but friction-feed is quickly growing in popularity for single-sheet "letter-quality" printers, such as ink jet. Electrophotographic printers use friction-feed techniques exclusively, but the EP paper handling system is far more involved than those used in moving-carriage printers .
The tractor-feed technique uses a continuous length of folded paper with holes evenly perforated along both sides. This paper is threaded into the paper transport, and inserted into sprocket wheels located just over the platen. Teeth on each sprocket wheel mesh exactly with holes in the paper. The platen and sprocket wheels are linked mechanically, so as the platen is advanced, both sprockets tum to pull paper through evenly. Some tractor-feed configurations can be threaded somewhat differently so that paper is pushed through the printer instead of pulled through. The friction-feed technique is effective for single sheets of paper. In most cases, any fresh, dry sheet of standard 20-lb bond xerography-grade paper will do, though ink jet printers might require specialized ink jet paper for best results. A sheet is started into the mechanism, then locked down against the platen using one or more pressure (or "pinch") rollers. Paper is now fixed in place. As the platen advances, friction between the platen and pinch rollers literally push the paper along. Chapter 10 discusses paper transports (and other mechanical systems) in more detail.
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